Level calibrated recordings of the Brood XIII 17-year periodic cicadas were made in June, 2007 in River Forest, Illinois. Brood XIII consists of three separate species: Magicicada cassini, M. septendecim, and the more rare M. septendecula. Power spectrum analysis of a 2 min segment of the recording shows a 5.95 kHz resonance with Q~6, indicative of cassini, and a 5 dB lower resonance at 1.35 kHz with a Q~13.5, indicative of septendecim. Since other researchers have estimated that the power of septendecim is 20 dB below that of cassini, the high level of the septendecim song suggests that 30 times more septendecim than cassini were singing. Computation of the sound pressure level shows a 2-5 dB variation with a period of 7.5 s, indicative of synchronized chorus singing of cassini.